MHC recognition in thymic development: distinct, parallel pathways for survival and lineage commitment

The molecular events triggered by MHC recognition and how they lead to the emergence of mature CD4 and CD8 lineage thymocytes are not yet understood. To address these questions, we have examined what signals are necessary to drive the development of CD8 lineage thymocytes in TCRalpha(-) mice in which TCR/MHC engagement cannot occur. We find that the combination of constitutive Notch activity and constitutive Bcl-2 expression are necessary and sufficient to allow the appearance of mature CD8 lineage thymocytes in TCRalpha(-) mice. In addition, Notch activity alone in TCRalpha(-) mice can induce the up-regulation of HES1, suggesting that thymocytes are competent to respond to Notch signaling in the absence of MHC recognition. These data indicate that survival and lineage commitment represent distinct, parallel pathways that occur as a consequence of MHC recognition, both of which are necessary for the development of mature CD8 lineage T cells.

Highly efficient selection of CD4 and CD8 lineage thymocytes supports an instructive model of lineage commitment

We undertook a kinetic analysis of the generation of mature T cells in TCR and coreceptor transgenic mice using BrdU labeling. We observed that the selection efficiency of mature CD4-CD8+ and CD4+CD8- thymocytes could be as high as 40% and 90% of CD4+CD8+ precursors, respectively. The surprisingly high efficiency of selection favors an instructional model of lineage commitment and is incompatible with a stochastic model in which the efficiency of selection would be no greater than 100% in both lineages combined.

The role of CD8 alpha' in the CD4 versus CD8 lineage choice

During thymic development the recognition of MHC proteins by developing thymocytes influences their lineage commitment, such that recognition of class I MHC leads to CD8 T cell development, whereas recognition of class II MHC leads to CD4 T cell development. The coreceptors CD8 and CD4 may contribute to these different outcomes through interactions with class I and class II MHC, respectively, and through interactions with the tyrosine kinase p56lck (Lck) via their cytoplasmic domains. In this paper we provide evidence that an alternatively spliced form of CD8 that cannot interact with Lck (CD8 alpha') can influence the CD4 vs CD8 lineage decision. Constitutive expression of a CD8 minigene transgene that encodes both CD8 alpha and CD8 alpha' restores CD8 T cell development in CD8 alpha mutant mice, but fails to permit the development of mismatched CD4 T cells bearing class I-specific TCRs. These results indicate that CD8 alpha' favors the development of CD8-lineage T cells, perhaps by reducing Lck activity upon class I MHC recognition in the thymus.

Regulation of T cell fate by Notch

The transmembrane receptor Notch participates in diverse cell fate decisions throughout embryonic development. Notch receptors and their ligands are expressed in the mammalian thymus, raising the possibility that Notch could regulate T cell fate decisions. Expression of a constitutively activated form of Notch in developing thymocytes causes thymocytes normally destined for the CD4 lineage to adopt the CD8 lineage instead. This suggests that Notch activity normally acts to direct CD4+CD8+ precursors to the CD8 lineage. The choice between CD4 and CD8 T cell fates is also controlled by MHC recognition during positive selection, implying that recognition of class I or II MHC might regulate Notch signaling. Possible models for the regulation of Notch by MHC recognition during CD4 versus CD8 lineage determination are discussed.

The cytoplasmic domain of CD8 beta regulates Lck kinase activation and CD8 T cell development

Previous studies have shown that CD8 beta plays a role in both enhancing CD8 alpha-associated Lck kinase activity and promoting the development of CD8-lineage T cells. To examine the role of this enhancement in the maturation of CD8-lineage cells, we assessed CD8 alpha-associated Lck kinase activity in both T cell hybridomas and thymocytes of mice expressing CD8 beta mutations known to impair CD8 T cell development. Lack of CD8 beta expression or expression of a cytoplasmic domain-deleted CD8 beta resulted in a severalfold reduction in CD8 alpha-associated Lck kinase activity compared with that observed with cells expressing wild-type CD8 beta chain. This analysis indicated a critical role for the cytoplasmic domain of CD8 beta in the regulation of CD8 alpha-associated Lck activity. Decreased CD8 alpha-associated Lck activity observed with the various CD8 beta mutations also correlated with diminished in vivo cellular tyrosine phosphorylation. In addition, analysis of CD8 beta mutant mice (CD8 beta-/- or cytoplasmic domain-deleted CD8 beta transgenic) indicated that the degree of reduction in CD8 alpha-associated Lck activity associated with each mutation correlated with the severity of developmental impairment. These results support the importance of CD8 beta-mediated enhancement of CD8 alpha-associated Lck kinase activity in the differentiation of CD8 single-positive thymocytes.

The alpha beta versus gamma delta T-cell lineage choice

During thymic development, immature T cells rearrange and express the genes encoding the T-cell antigen receptor and mature as either alpha beta or gamma delta lineage T cells. In the past year, advances have been made in understanding the role of individual components of the T-cell antigen receptor complex in the development of alpha beta and gamma delta lineage T cells. In addition, the transmembrane receptor Notch has recently been implicated as a new player in alpha beta versus gamma delta lineage determination.

Notch in vertebrates

Homologs of the Notch receptor and its ligands participate in cell fate decisions during vertebrate development. The past year has seen significant advances in knowledge of the role of Notch in Xenopus neuronal development and T-cell development and in our understanding of the Notch signalling pathway in vertebrates. Connections have also been discovered between alterations in Notch function and human disease.

Notch activity influences the alphabeta versus gammadelta T cell lineage decision

The choice between the alphabeta or gammadelta T cell fates is influenced by the production of functional, in-frame rearrangements of the TCR genes, but the mechanism that controls the lineage choice is not known. Here, we show that T cells that are heterozygous for a mutation of the Notch1 gene are more likely to develop as gammadelta T cells than as alphabeta T cells, implying that reduced Notch activity favors the gammadelta T cell fate over the alphabeta T cell fate. A constitutively activated form of Notch produces a reciprocal phenotype and induces thymocytes that have functional gammadeltaTCR gene rearrangements to adopt the alphabeta T cell fate. Our data indicate that Notch acts together with the newly formed T cell antigen receptor to direct the alphabeta versus gammadelta T cell lineage decision.

An activated form of Notch influences the choice between CD4 and CD8 T cell lineages

Notch is a transmembrane receptor that controls cell fate decisions in Drosophila and whose role in mammalian cell fate decisions is beginning to be explored. We are investigating the role of Notch in a well-studied mammalian cell fate decision: the choice between the CD8 and CD4 T cell lineages. Here we report that expression of an activated form of Notch1 in developing T cells of the mouse leads to both an increase in CD8 lineage T cells and a decrease in CD4 lineage T cells. Expression of activated Notch permits the development of mature CD8 lineage thymocytes even in the absence of class I major histocompatability complex (MHC) proteins, ligands that are normally required for the development of these cells. However, activated Notch is not sufficient to promote CD8 cell development when both class I and class II MHC are absent. These results implicate Notch as a participant in the CD4 versus CD8 lineage decision.

The cytoplasmic domain of CD4 promotes the development of CD4 lineage T cells

Thymocytes must bind major histocompatibility complex (MHC) proteins on thymic epithelial cells in order to mature into either CD8+ cytotoxic T cells or CD4+ helper T cells. Thymic precursors express both CD8 and CD4, and it has been suggested that the intracellular signals generated by CD8 or CD4 binding to class I or II MHC, respectively, might influence the fate of uncommitted cells. Here we test the notion that intracellular signaling by CD4 directs the development of thymocytes to a CD4 lineage. A hybrid protein consisting of the CD8 extracellular and transmembrane domains and the cytoplasmic domain of CD4 (CD884) should bind class I MHC but deliver a CD4 intracellular signal. We find that expression of a hybrid CD884 protein in thymocytes of transgenic mice leads to the development of large numbers of class I MHC-specific, CD4 lineage T cells. We discuss these results in terms of current models for CD4 and CD8 lineage commitment.

Truncated mammalian Notch1 activates CBF1/RBPJk-repressed genes by a mechanism resembling that of Epstein-Barr virus EBNA2

The Notch/Lin-12/Glp-1 receptor family participates in cell-cell signaling events that influence cell fate decisions. Although several Notch homologs and receptor ligands have been identified, the nuclear events involved in this pathway remain incompletely understood. A truncated form of Notch, consisting only of the intracellular domain (NotchIC), localizes to the nucleus and functions as an activated receptor. Using both an in vitro binding assay and a cotransfection assay based on the two-hybrid principle, we show that mammalian NotchIC interacts with the transcriptional repressor CBF1, which is the human homolog of Drosophila Suppressor of Hairless. Cotransfection assays using segments of mouse NotchIC and CBF1 demonstrated that the N-terminal 114-amino-acid region of mouse NotchIC contains the CBF1 interactive domain and that the cdc10/ankyrin repeats are not essential for this interaction. This result was confirmed in immunoprecipation assays in which the N-terminal 114-amino-acid segment of NotchIC, but not the ankyrin repeat region, coprecipitated with CBF1. Mouse NotchIC itself is targeted to the transcriptional repression domain (aa179 to 361) of CBF1. Furthermore, transfection assays in which mouse NotchIC was targeted through Gal4-CBF1 or through endogenous cellular CBF1 indicated that NotchIC transactivates gene expression via CBF1 tethering to DNA. Transactivation by NotchIC occurs partially through abolition of CBF1-mediated repession. This same mechanism is used by Epstein-Barr virus EBNA2. Thus, mimicry of Notch signal transduction is involved in Epstein-Barr virus-driven immortalization.

Functional commitment to helper T cell lineage precedes positive selection and is independent of T cell receptor MHC specificity

Thymocyte differentiation proceeds from double positive CD4+CD8+ to single positive T cells. It has been proposed that this process occurs by an instructive or a stochastic mechanism. In this report, we show that in recombination-deficient mice (RAG-1-I-) constitutive expression of a CD8 transgene allows maturation of CD4+(CD8tg+) cells, which express mature levels of a transgenic class I-restricted T cell receptor, F5. Rescued F5+CD4+(CD8tg+) cells have equivalent levels of T cell receptor expression as CD8end+ cells, respond to cognate antigen and, upon stimulation, they exhibit a phenotype characteristic of CD4+ helper T cells. These data are consistent with a model of differentiation that predicts that thymocytes become functionally committed to a helper or cytotoxic lineage before the final step of positive selection and independently of MHC specificity of their T cell receptor.

A role for the cytoplasmic tail of the beta chain of CD8 in thymic selection

The CD8 coreceptor plays a critical role in the recognition of foreign antigens by mature T cells and in the development of class I-restricted T cells. CD8 can be expressed on the surface of T cells as either a heterodimer composed of an alpha and beta chain, or as a homodimer composed of two alpha chains. In this report, we show that a CD8 beta transgene that lacks a cytoplasmic domain can suppress expression of wild-type endogenous CD8 beta and act as a dominant negative mutation. We show that this dominant negative CD8 beta transgene interferes with the development of mature CD8 T cells to different extents depending on the individual class I-restricted TCR. These data suggest CD8 beta plays a role in thymic development, and that different class I-restricted TCRs differ in their dependence on the cytoplasmic tail of CD8 beta.

Constitutive CD8 expression allows inefficient maturation of CD4+ helper T cells in class II major histocompatibility complex mutant mice

Although mature CD4+ T cells bear T cell receptors (TCRs) that recognize class II major histocompatibility complex (MHC) and mature CD8+ T cells bear TCRs that recognize class I MHC, it is possible that the initial commitment of an immature thymocyte to a CD4 or CD8 lineage is made without regard to the specificity of the TCR. According to this model, CD4+ cells with class I TCR do not mature because the CD8 coreceptor is required for class I MHC recognition and positive selection. If this model is correct, constitutive expression of CD8 should allow CD4+ T cells with class I-specific TCRs to develop. In this report, we show that mature peripheral CD4+ cells are present in class II MHC-deficient mice that express a constitutive CD8.1 transgene. These cells share a number of properties with the major class II MHC-selected CD4 population, including the ability to express CD40 ligand upon activation. Although mature CD4 cells are also detectable in the thymus of class II MHC mutant/CD8.1 transgenic mice, they represent a small fraction of the mature CD4 cells found in mice that express class II MHC. These results indicate that some T cells choose the CD4 helper lineage independent of their antigen receptor specificity; however, the inefficiency of generating class I-specific CD4 cells leaves open the possibility that an instructive signal generated upon MHC recognition may bias lineage commitment.

Selective events in T cell development

Somatic rearrangement of the genes encoding antigen receptors allows the mammalian immune system to produce receptors that can recognize virtually any foreign protein. This rearrangement process also generates nonfunctional antigen receptors as well as receptors that can recognize self-proteins. During thymic development, T cells go through a testing process which ensures that cells expressing useless or harmful antigen receptors do not mature. The selection of T cells in the thymus consists of several components driven by distinct recognition events that have distinct consequences for the cell. T cell selection appears to begin before the rearrangement process is complete. Evidence is emerging that T cells have a developmental checkpoint to make sure that the antigen receptor beta gene is successfully rearranged before development can proceed. This checkpoint also appears to be linked to the regulation of rearrangement and mechanisms to ensure that each cell expresses only one antigen receptor (allelic exclusion). After a T cell has successfully rearranged and expressed both its alpha and beta antigen receptor genes, a second phase of selection occurs. During this phase, T cells with self-reactive antigen receptors are eliminated by negative selection, and T cells that can recognize foreign peptides bound to polymorphic self-MHC molecules are selected to mature (positive selection. How these seemingly incompatible forms of selection both occur is a subject of considerable interest. Positive selection is linked also to the choice between the CD4 helper lineage and the CD8 cytotoxic T cell lineage; the mechanisms by which these two events are linked is an ongoing area of investigation.

Stochastic component to development of class I major histocompatibility complex-specific T cells

The mechanism by which an initially uncommitted cell chooses between alternative fates is a central issue in developmental biology. In the mammalian thymus, CD4 helper T cells and CD8 cytotoxic T cells arise from a common precursor that expresses both CD4 and CD8. The choice between the CD4 and CD8 lineage is linked to the specificity of the T-cell antigen receptor expressed by a thymocyte, but whether lineage commitment is stochastic or instructed has not been definitively resolved. We present evidence that expression of a constitutive CD8 transgene during thymic selection permits development of mature CD4 cells bearing the class I-restricted F5 T-cell antigen receptor. These results suggest that there is a stochastic component to the development of class I major histocompatibility complex-restricted T cells.

Tolerance and immune regulation

Understanding of both the phenomenon of immunological tolerance and the mechanisms by which it is achieved is rapidly advancing, as was evident from a recent meeting. This report summarizes key developments in both T- and B-cell biology.

Expression of CD4 in transgenic mice alters the specificity of CD8 cells for allogeneic major histocompatibility complex

We have generated a transgenic mouse line in which a CD4 transgene is expressed on a significant fraction of the mature CD8+ lymphocytes but is not expressed in the thymus. This provides an opportunity to examine the functional consequences of CD4 expression in a population of class I-selected CD8+ lymphocytes. CD8+ lymphocytes expressing the CD4 transgene proliferate in response to allogeneic class I and class II major histocompatibility complex, whereas CD8+ cells from control animals proliferate only to allogeneic class I gene products. These observations suggest that the ability of a T-cell population to react with class II allogeneic major histocompatibility complex is determined by the presence of CD4.

Infection of brain cells by diverse human immunodeficiency virus isolates: role of CD4 as receptor

Cell lines originally derived from malignant tumours of the brain were infected by diverse human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) isolates. By surface immunofluorescence it was shown that susceptible cells did not bear the CD4 antigen. They were also non-permissive for the formation of plaques by vesicular stomatitis virus pseudotypes and did not form syncytia with HIV-producing cells. Virus production was of low titre, and reverse transcriptase and the p24 antigen were consistently undetectable in the culture supernatants. Output virus could be detected by cocultivation with a sensitive T cell line, C8166, by the culture of supernatant medium with T cells and by detection of proviral HIV DNA after amplification. A higher multiplicity of input virus was required to establish a brain cell infection than was required for T lymphocytes or monocytes. Some HIV-susceptible brain cells contained mRNA for CD4 but infection was not blocked by anti-CD4 antibodies. Apparently HIV infection of these cells does not involve CD4 as the cellular receptor.

Fractional diffusion-limited component of reactions catalyzed by acetylcholinesterase

The values of kcat/Km for the reactions of four substrates, p-nitrophenyl acetate (PNPA), propionyl-beta-methylthiocholine (PrMSCh), 3,3-dimethylbutyl thioacetate (DBTA), and acetylthiocholine (AcSCh), with acetylcholinesterase were determined as a function of increasing viscosity (eta rel) in sucrose-containing and in glycerol-containing buffers. Glycerol, or possibly some contaminant of it, was found to be a nonspecific inhibitor and sucrose a nonspecific activator of the enzyme as reflected in the dependence of kcat/Km values measured for PNPA and PrMSCh upon the concentration of these reagents. The rates of reactions of these two substrates, the first neutral and the second cationic, are chemically limited rather than diffusion limited, and they thus serve as quantitative controls or internal standards to monitor the effects of the viscosogens on the enzyme, which are not related to diffusion. The additional effect on kcat/Km over the controls observed for the rapidly reacting substrates AcSCh (cationic) and DBTA (neutral) serves as a measure of the extent to which these values of kcat/Km measure diffusion-controlled processes. The reaction rate of DBTA with the enzyme is 24% diffusion controlled as measured in glycerol-containing buffers and 16-20% as determined in sucrose-containing buffers, while that for AcSCh is 100% (in glycerol) and 24-40% (in sucrose) diffusion controlled.

Blunt transection of the pancrease treated by distal pancreatectomy, splenic salvage and hyperalimentation. Four cases and review of the literature

The increasing awareness among surgeons of overwhelming postsplenectomy sepsis has led to new and innovative procedures to save the spleen. In pancreatic transection injuries (Type II)26 the classical treatment has been distal pancreatectomy and splenectomy. The opportunity to treat several patients with pancreatic transections sustained during blunt abdominal trauma lead to the review of the literature on the subject. Particular attention was paid to treatment of these injuries during distal pancreatectomy with splenic salvage, appropriate drainage, and hyperalimentation. Associated complications were likewise investigated and reviewed.

A survey of the haematological, nutritional and biochemical state of the rural elderly with particular reference to vitamin C

A survey of the rural elderly has been carried out to determine basic haematological and biochemical data, and to establish a pattern of living for this 'well' population. The blood parameters related to diet, such as haemoglobin, folate, cholesterol, vitamin C etc., show considerable changes with increasing age in the male but little significant change in the female. The vitamin C status for both sexes compares favourably with that reported by other workers studying the urban elderly. The biochemical and haematological data derived provide a normal range with which the 'sick' elderly can be compared. The subjects were generally active and independent in spite of (or because of) a somewhat Spartan existence.